1. Field of the Invention
The present invention relates to a method for forming a pigment containing film on a work-piece.
The present invention also relates to a circulatory coating apparatus used for applying a pigment containing coating in which the pigment is liable to coagulate.
Furthermore, the present invention relates to a pigment containing film obtained using this circulatory coating apparatus.
2. Description of the Related Art
In general, in precision application of coatings which are required to have a very precise film thickness, the dip coating method is often used. One method of dip coating that is often used, because it permits control of the concentration of the coating liquid, its viscosity and the level of the liquid surface, is the circulatory method, in which dipping of the work-piece that is to be coated into the coating liquid causes this liquid to overflow from the coating tank, the liquid then being recirculated. Rotary pumps and diaphragm pumps etc. are employed in this circulatory method as means for moving the coating liquid.
In recent years, in the field of paints and pigments, the object of coating has changed from simply improving the appearance and preventing rust, etc., to that of obtaining improved functionality. Examples that may be given of such improved functionality are recording coloring matters or display coloring matters used as electroluminescent materials or electrochromic materials, near infrared absorption coloring matter for optical discs, coloring materials for use in devices such as printers and copiers, such as coloring matter for color filters, organic photoconductive materials and pressuresensitive or heat-sensitive materials, and coloring materials for energy use, such as organic coloring matter for solar cells.
Such improved functionality is often attained, in particular in thin films from the submicron to 20 micron thickness range, by higher pigment loadings than in the case of ordinary coatings. It is therefore necessary to use a pigment-rich low viscosity dispersion. As a result, it is extremely difficult to maintain dispersibility of the pigment, due to problems such as sedimentation of the pigment and occurrence of flocculation of the pigment when subjected to slight shock.
In order to maintain dispersion of the pigment, addition of surfactants, as is done in the coating industry, has been considered. However, not only is considerable time required for selection of the surfactant, but also, the surfactant constitutes an impurity which has an adverse effect on improvement of functionality. Furthermore, the circulation of the coating liquid produces bubbles in the coating liquid, which result in formation of coating non-uniformities. Such coating non-uniformities cause variation of device characteristics.
Several methods which have been proposed for preventing such coating non-uniformity. They include: applying ultrasonic waves to the circulating coating liquid (Japanese Patent Disclosure (Kokai) No. 60-68081); applying a shearing force to the coating liquid immediately before coating, for example by stirring (Japanese Patent Disclosure (Kokai) Nos. 60-146238, 146239, 146240, 146241 and 146242) and similar methods in which the pigment is redispersed, and the method of capture dispersion of secondary pigment particles (Japanese Patent Disclosure (Kokai) Nos. 60-29752 and 29753). However, the methods of pigment redispersion have problems regarding treatment capability, and the method of capture dispersion of secondary pigment particles complicates control of the concentration of the coating liquid, because of gradual decrease in the P/B (pigment/binder resin) ratio.
In addition, preparation of the dispersion in the coating liquid is usually performed by means of ball milling or sand milling. However, it is impossible to avoid admixture of impurities originating from the grinder into the dispersion. As methods of removing these impurities, the magnetic filtering method (Japanese Patent Disclosure (Kokai) No. 60-208759) and the ultracentrifuge separation method, etc., have been disclosed. However, these require fairly complicated operations.